1. Field of the Invention
The present invention relates to an image reading apparatus adapted for use in copiers, facsimile apparatus or the like, and more particularly to such image reading apparatus utilizing solid-state image sensors.
2. Description of the Prior Art
The use of CCD image sensors or MOS image sensors for original reading in copiers or facsimile apparatus has been proposed in recent years.
Such an image sensor utilized for original reading is generally of an one-dimensional type capable of electrical scanning in the principal scanning direction to obtain density information of a one-dimensional image in the form of time-sequential signals. Auxiliary scanning is achieved by relative displacement of the original with respect to the sensor, thereby converting the pattern of a two-dimensional original into time-sequential electric signals, which are transmitted through a transmission line to other devices such as recorder a or memory for further processing such as printing or display.
The commercially available image sensor such as a CCD is constrained, in obtaining high-quality image signals at a high speed, by the limitation in number of pixels or picture elements of the image sensor. The highest number of pixels available in the currently commercialized one-dimensional sensors is on the order of 2048, which only provides a pixel density of about 6.9 pixels/mm in reading an original of A3 size (297.times.420 mm) even when the principal scanning is conducted parallel to the shorter edge thereof. An image sensor with a larger number of pixels, which would provide a higher pixel density, is technically difficult to realize and would be expensive even if made technically feasible.
In order to avoid the above-mentioned difficulty, it has been proposed to increase the number of pixels by, as shown in FIG. 1, arranging plural image sensors CCD1, CCD2 in the lateral direction. In such a case, as shown in FIG. 2, signal reading clock pulses from a pulse source 4 are supplied to the first image sensor CCD1 to read a half of an original 1 reflected through a mirror 2 and a lens 3-1. Upon completion of the image reading by the first image sensor CCD1, the clock pulses are supplied to the second image sensor CCD2 to read the remaining half of the reflected image. The signals read from two image sensors are simply sent out in succession to obtain time-sequential image signals OUT similar to those obtained from a single image sensor.
However, the reading clock frequency in the CCD or MOS image sensors has an upper limit ordinarily on the order of 2 MHz or of 10 MHz for high-speed devices. This fact renders the high-speed reading difficult with the increase of the number of image sensors and gives rise to a slower image processing such as image printing.
As an example, two CCD's each having 2048 pixels and driven with a clock frequency of 2 MHz require 2.048 msec for each principal scanning and provide a density of about 13.8 pixels/mm for the shorter edge of the aforementioned A3-sized original. To obtain the same pixel density of 13.8 pixels/mm in the auxiliary scanning direction, the scanning over the longer edge (420 mm) will take as long as about 12 seconds. In actual application for example in a copier, the situation is aggravated because each principal scanning requires an additional time interval in order to match the start of the scanning operation with the printing cycle of the copier.
In order to obtain high-quality image signals at a high speed avoiding the above-mentioned difficulty, there has further been propopsed the following process. In said proposed process, with plural image sensors laterally arranged as shown in FIG. 1, the original is divided along the principal scanning direction into plural sections, which are simultaneously read respectively by plural image sensors. In this manner each slit line of the original image is read as plurally divided sections.
The information thus read by plural image sensors is temporarily stored in corresponding plural buffer memories, and is sequentially taken out to provide continuous information for one slit line. It is also possible to conduct the original reading simultaneously with the signal read-out from the memory if plural buffer memories are switchably provided for each image sensor. Such original reading process provides an improved resolving power and enables a high-speed reading.
Said process is however still associated with a drawback that the images read by plural image sensors may show aberration at the junction of neighboring images. In order that the images read by adjacent image sensors are made continuous, each image sensor has to be positioned with a precision of 10.sup.-3 mm or better, and such precision requires a highly sophisticated technology to achieve and has a certain limit.
Also even if such precision is achieved, each image sensor may be displaced afterwards under due to the effect of temperature, humidity or mechanical vibration, and such displacement will be fatal in an apparatus requiring a high resolving power.